1. Field of the Invention
The present invention relates to a wiring board and a manufacturing method thereof, and more particularly to a multi-layer wiring board having multiple wiring layers using polyimide resin for inter-layer insulation, and a manufacturing method thereof.
2. Description of the Related Art
Multi-layer printed wiring boards have been used heretofore as the wiring boards on which to mount LSI chips.
A multiple printed wiring board has copper-sputtered stacked plates as a core and prepregs as an adhesive for connection between the copper-sputtered stacked plates, and the copper-sputtered stacked plates and prepregs are alternately stacked and integrated by thermal press processes.
Electrical connections between the copper-sputtered stacked plates are achieved by forming penetrating throughholes with a drill after the copper-sputtered stacked plates and prepregs are integrated, and coating the inner walls of the penetrating throughholes with copper.
Furthermore, in recent years, multi-layer wiring boards using polyimide resin over ceramic substrates for inter-layer insulation have come to be used as wiring boards for high performance computer use, where greater density than that of multi-layer printed wiring boards is required.
A polyimide ceramic multi-layer wiring board which has polyimide resin insulating layers and multiple wiring layers is formed by the repetition of following two processes.
Polyimide resin insulating layers are formed by supplying polyimide precursor varnish over a ceramic substrate, drying it and forming via holes in this coating film over the ceramic substrate.
Multiple wiring layers are formed by photolithographic methods, vacuum evaporation methods, and metal plate methods.
On the other hand, there also is a method to form a polyimide ceramic multi-layer wiring board by forming conductor lines as a signal layer over polyimide sheets, aligning these sheets over a ceramic substrate, and successively alternately stacking and pressing sheets over the ceramic substrate.
However, the above-mentioned multi-layer wiring boards have some following disadvantages.
For the above-mentioned multiple printed wiring board, since electrical connections between the copper-sputtered stacked plates are achieved by penetrating throughholes which are formed through from the top printed wiring board to the bottom printed wiring board with a drill, it is impossible to form physically minute penetrating throughholes in the signal wiring layer part of the multiple printed wiring board, and accordingly it is impossible to form a lot of conductor line between the penetrating throughholes over the copper-sputtered stacked plates.
Furthermore, one penetrating throughhole is always needed for electrical connection between every stacked plates, so that the capacity to accommodate signal conductor lines is reduced by an increase in the number of stacked layers, resulting in the disadvantage that it is difficult to form multiple printed wiring board having high wiring density.
On the other hand, since the above-mentioned polyimide ceramic multi-layer wiring board is always formed by the repetition of the processes of supplying polyimide precursor varnish over the ceramic substrate, drying it, forming via holes in the coating film and curing it as many times as the number of polyimide insulating layers stacked over the ceramic substrate, the stacking process of the polyimide resin insulating layers takes an extremely long time.
Moreover, since the above-mentioned polyimide resin insulating layers are always formed by the repetition of the processes of supplying polyimide precursor varnish over the ceramic substrate, drying it, forming via holes in the coating film and curing it, the thermal stress of the curing process which is repeated many times is applied on polyimide resin in the lower layer part of the polyimide resin insulating layers.
Therefore, it is difficult to improve the manufacturing yield of polyimide resin insulating layers because of deterioration of lower layer part of the polyimide resin insulating layers due to thermal stress of curing process.
On the other hand, since the above-mentioned polyimide ceramic multi-layer wiring board is always formed by forming conductor lines as a signal layer over polyimide sheets, aligning these sheets over a ceramic substrate, and successively alternately stacking and pressing sheets over the ceramic substrate, the thermal stress of stacking and pressing processes is applied on polyimide resin in the lower layer part of the polyimide resin insulating layers many times, and the polyimide resin in the lower layer part of the polyimide resin insulating layers is deteriorated and the stacking process of the polyimide resin insulating layers takes an extremely long time as a result.